We are not accepting a 90-95% slab where product goes in at 80%.
The GC had an independent test with a GE Hygromaster and it read under 70%. What gives here? For fun, I took a calcium chloride test got a 3.61.
Whats wrong with the GE Hygromaster? Someone told me it did not satisfy the 2170?
The work will go to our competition. They will perform based on the independent's test results, with knowledge of these issues. Any comments would be appreciated.

This is the very reason for all the confusion and failures in the industry.

Without getting into too much background of in-situ testing and what it can tell us, let me just say the test itself is not difficult but it can be improperly conducted by someone who doesn't understand what the test is supposed to show us.

What we need to know is the RH of the concrete slab at a depth of 40% of its thickness. A probe in a loose fitting sleeve actually reads from a larger horizon than right at 40%. Including the much drier top third of the slab will give false low readings.

Drilling the hole too deep or too shallow, a sloppy fitting sleeve, not acclimating the probe for a MINIMUM of 60 minutes per hole, calibration verification or lack of, debris in the hole.... all these things and more can effect the readings on most RH testing systems. In fact I have even watched sunlight warm a probe shaft and throw readings off.

That said, I think it is wise you do not do the work if you cannot trust the substrate. As more and more owners get bitten by installing on wet slabs, proper testing and selling the benefits of such are becoming increasingly easier. You said you did a CaCl test (another highly questionable test) and it read a bit high. How can the installer justify his installation when the CaCl test is elevated?

The Hygromaster sleeves that I have seen do not meet ASTM 2170 standards. They are vented on the sides and read an average moisture content. This will give you a false low reading. ASTM requires you to read the RH at 40% of slab depth not an average. I have had success using the Hygromaster with Vaisala sleeves (hollow tube that is cut to the depth of the hole).

Ask the GC if the Hygromaster sensor had been checked for calibration in the last 30 days (an ASTM requirement). Using a salt and water calibration test I have found the GE sensors to vary as much as 10 percentage points.